A number of cellular communications systems are currently being installed to provide communications facilities to mobile terminals. In such systems, mobile terminals are served by base stations via a radio or air link, each base station defining a system cell. In order that a subscriber may be identified as an authorised system user and located so as to receive calls or other system services, the system is provided with a central database containing subscriber information which can be accessed as required. In the standardised European system commonly referred to as the GSM system, this central store is known as the home location register (HLR). The number of subscribers to mobile communications systems is rapidly increasing, and service providers are becoming concerned that their systems should have sufficient capacity to accommodate these subscribers. A particular problem is the need to store information relating to subscribers on the system home location register, as it will be appreciated that the installed storage capacity of a home location register is finite. Although systems have been designed to allow for future expansion, the unexpected growth in the number of mobile subscribers will soon exceed the storage limits of the present home location registers. One solution to this problem is the installation of a larger home location register to replace existing equipment. However, this is generally undesirable both on the grounds of cost and the possible system disruption that can occur during installation. An alternative solution is the provision of second home location register to which new subscribers are allocated when the existing register becomes full thus effectively doubling the storage capacity. It has been found however that this introduces a routing problem as each request then requires the inclusion of a code indicating the identity of the particular home location register to which the subscriber has been allocated. This requires modification of the network routing tables to accommodate this additional information and can lead to a complex routing which is difficult to manage.
Our co-pending application Ser. No 08/884309 discloses a mobile network arrangement incorporating two or more home location registers. In this arrangement, a primary home location register (HLR) is provided with one or more cascaded home location registers to which some of the subscribers are allocated. This permits sharing of the workload in processing the subscriber information requests that are involved in the setting up of calls. Routing of requests to the correct HLR is simplified by routing all such requests to the original or master HLR and forwarding only those requests for which data is stored on one of the back-up or cascaded HLRs. This avoids the need for changes in the existing system routing plan when new subscribers are added or when a subscribers details are transferred from one HLR to another.
The arrangement and method described in Ser. No 08/884309 addressed the problem of introducing a second or auxiliary home location register to an existing system to meet subscriber growth without the need for changes to existing network routing tables.
In a mobile telephone network, such as a GSM network, the routing of messages to a home location register is normally based on the subscriber number, the IMSI or the MSISDN. The IMSI, which is provided on the subscriber's SIM card, is an internal network identification of the subscriber and the MSISDN is the subscriber's diallable phone number. Ideally, there will be a co-ordination between the allocation of a subscribers MSISDN and IMSI.
In a network having more than one home location register, the subscribers will generally be partitioned logically between the registers. In order for a subscriber to gain access to the network, he must first obtain a SIM card, usually from a mobile phone retailer. However, the distribution of SIM cards to retailers and their subsequent allocation to subscribers cannot be strictly controlled, and the direct co-ordination between IMSIs and MSISDNs may be lost.
Ideally, network translations should route IMSI and MSIDN based messages to the same HLR. However, because the co-ordination of these two numbers cannot be guaranteed owing to the aforementioned difficulty of controlling the retail outlets issuing SIM cards to customers, it is possible for an IMSI based subscriber message to be routed to one HLR and a MSIDN message for the same subscriber to be rerouted from that HLR to another HLR. Further, where a subscriber's details are not stored on any system HLR, there is a risk that messages relating to that subscriber will be perpetually forwarded between HLRs.